The present invention relates to a moisture-sensing hygrometric element which measures the humidity of the atmosphere by performing a humidity-current or humidity-voltage conversion resulting from the electrolysis of water.
In the past, ceramics and polymers have generally been used as moisture-sensing element materials for measuring humidity. Ordinarily, such elements have used a detection means based on a resistance system or an electrostatic capacity system. However, elements of both of these types have been somewhat unsatisfactory in terms of ease of handling, high-precision measurement across a broad humidity range and useful life. Few of these elements can be used in a corrosive atmosphere.
A method which utilizes the electrolysis of water to measure the humidity of a gas has been disclosed in U.S. Pat. No. 4,083,765. Specifically, a sensor is proposed which uses a tube made of a fluororesin-type cation exchange resin containing sulfonic acid groups as ion exchange groups, and in which platinum is caused to contact the internal and external surfaces of the tube in coil form as electrodes. Since the materials used are intrinsically superior in terms of corrosion resistance, this sensor can be used in a corrosive atmosphere. A linear correlation between humidity and current across a broad humidity range is provided.
The use of an assembly in which electrodes are bonded to both surfaces of a fluororesin ion exchange resin membrane as a moisture-sensing element has been proposed in Japanese Laid-Open Patent Application (Kokai) No. 60-36947.
The formation of an electrode pattern on the surface of an ion exchange resin membrane using a resist ink in order to increase the exposed surface area ratio of the ion exchange resin membrane has been proposed in Japanese Laid-Open Patent Application (Kokai) No. 61-264247.
In the case of the sensor disclosed in U.S. Pat. No. 4,083,765, a certain minimum film thickness is required in order to provide the fluororesin-type cation exchange resin membrane tube, which constitutes a solid polymer electrolyte, with adequate mechanical strength. As a result, the capacity of the tube for containing water is very large, so that the response to humidity changes is slow in a static atmosphere. Furthermore, it is also structurally difficult to maintain a uniform contact between the tube and the electrode wire coils in long-term use. Since the output current changes with a change in the contact resistance, the reliability of the sensor is poor. In addition, in cases where the contact is poor so that the contact resistance is large, the current value is large when a high humidity is measured, so that heat is generated in the contact area, causing the tube to deteriorate.
In the case of the aforementioned moisture-sensing element disclosed in Japanese Laid-Open Patent Application (Kokai) No. 60-36947, the abovementioned problem of the contact resistance is solved. However, since virtually the entire surface of the membrane is covered by the electrodes, the exposed portion of the ion exchange membrane, which acts as the moisture-sensing element, is small so that the response to fluctuations in humidity is relatively poor.
Because oxygen and hydrogen are generated as electrolysis products at the electrodes, the equilibrium of the surrounding water vapor is disturbed, and the output tends to be unstable.
In the case of the sensor disclosed in Japanese Laid-Open Patent Application (Kokai) No. 61-264247, these problems are ameliorated by using a resist ink. In this case, however, the assembly must be formed by a complicated chemical plating process which includes the formation and removal of a resist ink pattern. As a result, it is difficult to obtain reproducible moisture-sensing characteristics, and it is also difficult to improve the yield and reduce manufacturing costs.